Multicomponent NMR Titration for Simultaneous Measurement of Relative pK_as

Abstract

An NMR titration method has been developed to simultaneously measure the difference in acid dissociation constants (delta pKa) of two or more compounds with high precision and accuracy. The delta pKa between the conjugate acids of the two stereoisomers of 4-tert-butylcyclohexylamine 1 was determined in a single 1H NMR titration experiment. A mixture of the two stereoisomers was titrated with DCI in a 3:1 (v/v) mixture of CD3OD/D2O. From the variations of the H1 chemical shifts the ratio of the acidity constants was determined. The trans stereoisomer 1t was found to be the more basic by 0.121 +/- 0.002 pK unit. A repeat titration in DMSO-d6 also found 1t to be the more basic, by 0.217 +/- 0.003 pK unit. The delta pKa between the two stereoisomers of 4-tert-butylcyclohexanecarboxylic acid 2 was determined in both of these solvents using 1H and 13C NMR. Thermodynamic parameters delta delta H degree and delta delta S degree were evaluated from the temperature dependence of delta pKa. To further demonstrate the utility of this method, the delta pKas of the conjugate acids of the four stereoisomers of 2-decalylamine 3 were determined in a single 1H NMR titration experiment. The cis,cis stereoisomer (3cc) was found to be the most basic, with the cis,trans (3ct), trans,cis (3tc), and trans,trans (3tt) less basic by 0.012 +/- 0.003, 0.037 +/- 0.004, and 0.141 +/- 0.005 pK unit, respectively. A second four-component titration was also performed with the two 4-tert-butylcyclohexylamines (1) and the two trans-2-decalylamines (3tc, 3tt).

Determination of dissociation constants of folic acid, methotrexate, and other photolabile pteridines by pressure-assisted capillary electrophoresis

“…In the NMR measurements, the pteridine in question was cotitrated with Tris. Thus, the difference-sensitive evaluation method of Perrin and Fabian could also be applied [45,48]. This relies on the fact that chemical shifts are usually more accurately accessible than sample pH values.…”

pH-Free Measurement of Relative Acidities, Including Isotope Effects

“…Second order rate constants for thermoneutral exchange are at least 3x10 8 M 1 s 1 in water and are reduced only to ~5x10 6 M 1 s 1 for some hindered amines in CDCl 3 ( 45 ). These values mean that peak broadening from insufficiently rapid exchange is negligible even at millimolar concentrations, although this may not be the case for some proteins in solutions lacking sufficient buffer to mediate proton transfer ( 67 ).…”

“…8, relating observed chemical shifts to the desired K. This trivial linearization has been dignified as the Perrin Fabian method. Then from the observed chemical shifts  a and  b during the titration of a mixture of compounds, the ratio of acidity constants can be calculated from a plot of ( b  B°) ( AH +  a ) versus ( a  A°) ( BH +  b ), which ought to be a straight line, with slope K and zero intercept ( 8 ). Therefore, the beginning chemical shifts of the neutral compounds ( B° and  A°) and the ending chemical shifts of the ions ( BH + and  AH + ) must be known to accurately determine the ratio of acidity constants using this methodology.…”

No Contribution of an Inductive Effect to Secondary Deuterium Isotope Effects on Acidity

“…A powerful NMR spectroscopy titration method makes it possible to measure relative acidities, [18] and thus to measure isotope effects on acidity with great accuracy. The method involves titrating a mixture of isotopologues (which differ only in isotopic composition, or number of isotopic substitutions) with small aliquots of base.…”